1. Field of the Invention
The present invention relates to piezoelectric transducers whose vibrating element is formed of a piezoelectric material layer membrane having electrodes and being held at its periphery in a capsule case.
2. Description of the Prior Art
This type of capsule has been known for many years and is described for example in the documents GB-A-2 046 554, JP-A-58 202699 or JP-A-60 199298.
In these known capsules, the membrane is formed of a thin disk to which is fixed a piezoelectric ceramic layer covering the central zone of the disk, the two faces of the piezoelectric ceramic layer being metallized so as to form respectively an internal electrode applied against the thin disk and an external electrode. The membrane is housed in a case formed of a base body and a lid fitting one on the other along a closed generally circular periphery. The base body forms a first wall substantially parallel to the disk and defining with the disk a rear acoustic cavity. The lid forms a second wall substantially parallel to the disk and defining with the disk a front acoustic cavity. The case comprises a supporting surface, for example in the form of an inner peripheral step, against which the periphery of the disk is held applied by holding means. Electric connection means provide the respective electric connections of one and other of the electrodes, with a first and second external connection terminal of the case.
In known capsules, the membrane is held mechanically in position with respect to the case by a rigid or resilient element bearing along the periphery of the membrane disk.
Thus, in the document GB-A-2 046 554, the periphery of the disk bears on a peripheral supporting surface of the base body, against which it is held by a peripheral O-seal inserted between the lid and the disk. In the documents JP-A-58 202699 or JP-A-60 199298, the disk is also held against a peripheral supporting surface of the case, against which it is urged by a peripheral O-seal inserted between the disk and the case.
The inventors have discovered that the presence of such O-seals in a capsule has drawbacks during large-scale production; in fact, known seals usable for large scale production are generally of dimensions which are not very regular, which induces defects in the evenness of the surfaces holding the piezoelectric membrane and variations in the acoustic qualities of the capsules thus obtained. The defects related to the presence of O-seals also result from the variation of thickness of the other stacked parts forming the capsule, particularly the base body and the lid, creating a nipping force on the periphery of the disk which is difficult to control. The mechanical holding means of the invention avoid such variations, and hold the membrane in position without using an O-seal.
According to another object of the invention, the new capsule structure substantially increases the possibilities of deformation of the peripheral zone of the disk, so that the electroacoustic qualities of the capsule are substantially increased.
When seeking to do without the presence of an O-seal or peripheral fitting of the membrane, the difficulty arises from the fact that the seal or fitting, in known capsules, not only hold the membrane against the step of the case but further provide sealing between the front acoustic cavity disposed in front of the membrane and the rear acoustic cavity disposed at the rear of the membrane. For that, the particular means of the invention, for holding the membrane mechanically in the case, provide at the same time sufficient sealing between the front acoustic cavity and the rear acoustic cavity.
Such sealing is further obtained, according to the invention, in a structure which substantially increases the possibilities of deformation of the peripheral zone of the disk. A priori, the two functions lead to contrary requirements, which the invention reconciles.
The invention further provides particular embodiments leading to the general improvement of the electroacoustic qualities of the capsule. In particular, attempts have been made to distribute the natural mechanical vibratory frequencies of the different elements of the capsule, so as to obtain the flattest possible response curve in the usual operating frequency range.
Another effect sought is to obtain a good compromise between the acoustic effect of the capsule and its size, particularly its thickness.